Many congresses are accompanied by tragic events by the nature of their meaning and the fact that serious conflicts need resolution . Unfortunately they are hotspots for tragic events, especially where the established authority is being challenged by the masses.
They are in the most part formal and will have representation from several groups, parties, unions , religious leaders and so on. They can easily escalate .
Such things have happened in the west in the past and still do to this day, the UK government has changed the law on groups or protesters gathering to try and avoid such large groups.
The G8 summit gets lots of public protesters against capitalism for example.
Throw in , sexual freedom, famine, womens rights , religion, workers rights, taxes etc, then there's always the potential for tragic events, historically the UK is littered with them.
I can't see the link to a group of hikers doing their pass time to get a certificate and see how this would be a threat to any soviet faction/union/ party whatsoever that some authority would order their deaths during the 21st anniversary.
It's in the middle of winter, several other groups of hikers are doing the same , there's Mansi roaming and moving throughout the trails making their living.
I believe it's well known that many tourist trips ended in injuries or fatalities, Dyatlov pass is not unique in this regard.
I do not see what message could be given if they were killed, what would be achieved by their murder ? They are not storming the winter palace with some ideologies, they are in the middle of nowhere, talking stories of love and music, collecting stones and taking photos . They were out of the way of any potential challenge to ,or celebration of, the 21st Congress .